ABSTRACT Cellular senescence limits the proliferation of human cells and can be induced by a variety of cellular alterations, both intrinsic and extrinsic. Senescent cells accumulate in human tissues, including the prostate, with increasing age. These senescent cells have altered function, including increased expression of proinflammatory cytokines that can alter the function of adjacent cells. Benign prostatic hyperplasia (BPH) is the single most common pathology of aging men. Based on our published studies we hypothesize that senescence of a subset of epithelial cells in BPH tissue leads to release of cytokines and growth factors that, through direct and indirect actions, drives increased proliferation of adjacent non-senescent epithelial cells and stromal cells and ultimately prostatic tissue growth in aging men. We propose to characterize the mechanisms by which cellular senescence can promote the development of benign prostatic hyperplasia. Two Specific Aims are proposed. In Specific Aim 1, we will examine the underlying cellular alterations leading to prostatic epithelial senescence in vitro and in vivo; determine the types of cytokines and growth factors expressed by senescent epithelial cells in vitro; evaluate whether these same proteins are expressed at increased levels in BPH tissue in vivo and quantitatively evaluate the extent to which there is coexpression of these cytokines and growth factors at the cellular level in vivo with markers of senescence, including key cell cycle regulator proteins such as p21 and p16. In Specific Aim 2 we will use primary cultures of prostatic epithelial and stromal cells, the reactive stroma model system and transgenic models to examine the biological activities of the identified cytokines/growth factors and model potential autocrine and paracrine activities of those factors that are increased in BPH in vivo. In addition, we will establish a transgenic mouse model of epithelial senescence and examine the biological impact of epithelial senescence in this mouse model. Benign prostatic hyperplasia causes considerable morbidity in older men, with up to 30% of men requiring treatment for this condition, and with more than one billion dollars spent on the medical and surgical treatment of this disease annually. These studies will make a fundamental contribution to our understanding of the role of cellular senescence in the pathogenesis of this common disease and lead to more effective preventive treatments and medical therapies.